Fluid conditioning systems are known in the art for providing fluid in a certain desired range of temperature, pressure and humidity. Such systems are particularly useful in providing conditioned air in aircraft.
Fluid conditioning systems are known in the art in which water is condensed from the working fluid, such as air, while at high pressure. U.S. Pat. No. 4,198,830 of Campbell discloses such a system with the addition of a reheater heat exchanger. In the Campbell patent, moist air is introduced into the reheater heat exchanger at relatively high pressure and warm temperature, and is cooled therethrough below the dew point. The air then flows through a first set of finned passageways of a condenser heat exchanger. The pressure does not change significantly, but a relatively large amount of water is condensed from the air and drained by a water separator, such as a water trap.
The air is then caused to reflow through the reheater exchanger where it is reheated and passed to an expansion turine. The purpose of reheating the air is two-fold: to re-evaporate any residual moisture in the air before entering the turbine, and to increase the total turbine and cycle efficiency by increasing the temperature of the air at the turbine entrance, thereby increasing the enthalpy for enhanced power extraction by the turbine. The air thus enters the turbine at relatively high pressure and warm temperature, with entrained water vapor but with virtually no entrained liquid water. During expansion in the turbine, a major portion of the remaining water vapor condenses.
After expanding in the turbine, the air is caused to flow through a second set of finned passageways in the condenser heat exchanger. This second set of passageways is substantially perpendicular to the first set. The condenser heat exchanger is a cross-flow type of exchanger well known in the art. Air exits the turbine and enters the condenser at low temperature and low pressure, with entrained liquid water. This cold air is used in the condenser heat exchanger to absorb heat from the air flow in the abovementioned first set of finned passageways, causing a large portion of the entrained water vapor to condense. Air exits the second set of finned passageways to a user, such as a cockpit of an aircraft.
The temperature of the condenser heat exchanger metal, at least near the cold side inlet facing the exit of the turbine, nay be below zero degrees Celsius under some extreme conditions. A problem of the above described installation is that at these extreme conditions, ice particles or snow present in the cold air exiting the turbine may strike and adhere to the cold inlet side if the condenser heat exchanger. As a result, ice and snow may build up on the cold inlet face. In addition, the entrained moisture in the hot air flowing through the condenser in the proximity of the cold inlet face may freeze on the surface of the finned passageways. Thus, ice deposition may block air flowing through both the cold and hot sides of the condenser heat exchanger.
U.S. Pat. No. 4,246,963 of Anderson addresses the problem of ice formation by passing hot fluid through hollow closure bars on the condenser heat exchanger near the inlets to the passageways of the cold air coming from the turbine.
U.S. Pat. No. 4,352,273 of Kinsell et al uses the heated closure bar idea of Anderson, and also uses a bypass in the middle of the condenser heat exchanger to ensure that an adequate supply of conditioned air is supplied to the user despite any ice formation.
A problem of the Anderson and Kinsell patents is that the closure bars add to the cost and complexity of the fluid conditioning system. Moreover, changes in turbine discharge velocities can cause flow and temperature stratification in the cold side inlet of the condenser heat exchanger. U.S. Pat. No. 5,025,642 of Brunskill et al tries to solve the stratification problem by using a back pressure plate to minimize flow velocity stratification and a bypass to produce a relatively predictable bypass flow ratio regardless of flow velocity stratification. Of course, the first problem of cost and complexity is not addressed but rather aggravated.
Another problem is that the referenced prior art tries to defeat ice formation only at the cold side inlet of the condenser heat exchanger. However, ice may form at other portions of the condenser, including the passageways for flow of air from the reheater heat exchanger. Heating closure bars at the cold side inlet and bypass devices only help deice flow from the turbine through the condenser heat exchanger, but have little to do with deicing the reheater flow.
Furthermore, ice formation causes partial blockage of finned passageways and causes a larger portion of the air to flow through the bypass. More air flowing through the bypass means that less water is condensed and then separated by the water separator, so that more water is added to the air flow reaching the turbine, ultimately increasing ice formation at the condenser heat exchanger inlet. This phenomenon has been confirmed during system tests of equipment built in accordance with the prior art, in which large amounts of ice formed in the condenser heat exchanger. In extreme conditions, almost all of the finned passageways were blocked.